1. Field of the Invention
The present invention relates generally to the field of integrated circuit testing and, more specifically, the present invention relates to endpoint determination when milling an integrated circuit.
2. Description of the Related Art
Once a newly designed integrated circuit has been formed on a silicon substrate, the integrated circuit must be thoroughly tested to ensure that the circuit performs as intended. Any portion of the integrated circuit that does not function properly must be identified so that it can be fixed by modifying the design of the integrated circuit. This process of testing an integrated circuit to identify problems with its design is known as debugging. After debugging the integrated circuit and correcting any problems with its design, the final fully functional integrated circuit designs are used to mass produce the integrated circuits in a manufacturing environment for consumer use.
During the debugging process, it is sometimes necessary to probe certain internal electrical nodes in the integrated circuit in order to obtain important electrical data from the integrated circuit, such as for example voltage levels, timing information, current levels and thermal information. One method of probing integrated circuit nodes to obtain the important electrical data involves milling the substrate of the integrated circuit to expose an active diffusion region of interest. Once exposed, the active diffusion region of interest may be probed directly to obtain electrical data using a variety of techniques.
FIG. 1 is an illustration of a cross-section of active diffusion region 103 disposed in an integrated circuit substrate 101. As shown in FIG. 1, active diffusion region 103 is bordered by isolation oxide regions 105 and 107. Active diffusion region 103 is coupled to ground 113 through metal interconnect 111. Similarly, substrate 101 is coupled to ground 115.
Assuming it is desired to collect electrical data from active diffusion region 103, one method of probing active diffusion region 103 involves exposing active diffusion region 103 by milling substrate 101 to create opening 109. Once opening 109 is created in substrate 101, active diffusion region 103 is exposed and may then be probed using a variety of techniques.
An important issue that must be considered when milling an integrated circuit substrate 101 is endpointing, which is determining precisely when to discontinue the milling procedure. For instance, if on the one hand milling is discontinued prematurely, then opening 109 would be too shallow and therefore not adequately expose active diffusion region 103. If on the other hand the milling process is not stopped in time, then opening 109 may be too deep and result in active diffusion region 103 being unintentionally damaged or destroyed.
Present day techniques for endpointing, or determining when to stop milling, involve utilizing a focused ion beam (FIB) milling tool, which focuses an ion beam on the surface of substrate 101 during the milling procedure. Since isolation oxide regions 105 and 107 are dielectric regions and since there is no path to ground from these regions, the focused ion beam of the FIB milling tool charges isolation oxide regions 105 and 107 once they are reached and become exposed. In contrast, active diffusion region 103 conducts charge and therefore provides a path to ground 113 through metal interconnect 111. The same holds true for substrate 101 in that there is a path to ground 115 for any charge received from the ion beam. Therefore, active diffusion region 103 and substrate 101 do not become charged from the focused ion beam of the FIB milling tool when exposed.
Accordingly, an imaging detector included in the FIB milling tool can image the substrate during milling and detect when isolation oxide regions 105 and 107 become charged and are therefore reached. For instance, if the FIB milling tool is in electron contrast mode, isolation oxide regions 105 and 107 will appear dark when charged relative to other grounded neighboring regions. If, for example, the imaging detector of the FIB system is in ion contrast mode, then oxide regions 105 and 107 will appear light when charged relative to the other grounded neighboring regions.
Since active diffusion region 103 is disposed near isolation oxide regions 105 and 107 as shown in FIG. 1, milling can be stopped when isolation oxide regions 105 and 107 are reached with the FIB tool. When milling is completed, active diffusion region 103 is effectively exposed for probing purposes.
One disadvantage of the present day technique illustrated in FIG. 1 for exposing an active diffusion region in an integrated circuit substrate is that it is extremely important to accurately determine the location to mill in the substrate when exposing an active diffusion region of interest. To illustrate, FIG. 2 is an illustration of a cross-section of an active diffusion region 203 in an integrated circuit substrate 201. Active diffusion region 203 is bordered by isolation oxide regions 205 and 207. The integrated circuit cross-section illustrated in FIG. 2 also includes a transistor, which includes active diffusion regions 219 and 221 disposed in well region 217 with a gate electrode 223. Also illustrated in FIG. 2 is another transistor that includes active diffusion regions 229 and 231 disposed in well region 227 with gate electrode 233. Active diffusion region 203 is coupled to ground 213 through metal interconnect 211. Integrated circuit substrate 201 is coupled to ground 215. Well regions 217 and 227 are coupled to ground at locations 225 and 235 respectively.
Assuming it is desired to collect electrical data from active diffusion region 203, integrated circuit substrate 201 is milled to expose active diffusion region 203 for probing. Assuming further that there is a slight inaccuracy in the milling location selected to expose active diffusion region 203, there is a risk of destroying neighboring features in the integrated circuit substrate 201. For instance, as shown in FIG. 2, it is desired to expose active diffusion region 203 and opening 209 is therefore milled. As also shown in FIG. 2, opening 209 is slightly offset and well region 217 is therefore unintentionally milled as well.
By using present day techniques for endpointing, the milling process is not stopped until isolation oxide region 205 is reached as indicated by the imaging detector of the FIB milling tool. As shown in FIG. 2, well region 217 is deeper than isolation oxide region 205. Consequently, a significant portion of well region 217 is milled before the milling process is discontinued as indicated by isolation oxide region 205 when creating opening 209. In some instances, a severe amount of well region 217 is removed before milling is discontinued and the associated transistor is destroyed as a result. Thus, isolation oxide regions 205 and 207 sometimes do not adequately indicate when the milling procedure should be stopped.
It is appreciated that it may be possible to detect changes in diffusion doping chemistry to determine when to discontinue the FIB milling process by means of a mass spectrometer analysis of the chamber exhaust. However, the low secondary ion yield during FIB milling process results in a poor signal to noise ratio and therefore does not provide an accurate endpoint determination. Furthermore, it a noted that this type of technique is also very costly.
Therefore, what is needed is a method and an apparatus for endpoint determination when milling an integrated circuit substrate. Such a method would provide a technique for accurately determining when to stop the milling process when a diffusion boundary is reached. Such a method and an apparatus would reduce the risk of inadvertently destroying neighboring features of an integrated circuit diffusion being exposed during the milling process. Such a method and apparatus should be easily adaptable to existing milling tools, require little if any modifications to the existing equipment and be extremely low cost.